Proton-conducting structure and method for manufacturing the same

ABSTRACT

A proton-conducting structure that exhibits favorable proton conductivity in the temperature range of not lower than 100° C., and a method for manufacturing the same are provided. After a pyrophosphate salt containing Sn, Zr, Ti or Si is mixed with phosphoric acid, the mixture is maintained at a temperature of not less than 80° C. and not more than 150° C., and thereafter maintained at a temperature of not less than 200° C. and not more than 400° C. to manufacture a proton-conducting structure. The proton-conducting structure of the present invention has a core made of tin pyrophosphate, and a coating layer formed on the surface of the core, the coating layer containing Sn and O, and having a coordination number of O with respect to Sn of grater than 6.

This is a continuation-in-part application under U.S.C 111(a) of pendingprior International application No. PCT/JP2009/006180, filed on Nov. 18,2009, which in turn claims the benefit of Japanese ApplicationNo.2008-297541 filed on Nov. 21, 2008, the disclosures of whichApplication are incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a proton-conducting structure thatcontains a pyrophosphate metal salt and is superior in protonconductivity, and a method for manufacturing the same.

BACKGROUND ART

Since a proton conductor conducts only proton, and has electricalproperties as an insulator, it has been used as an electrolyte of fuelcells. Among such proton conductors, solid electrolytes composed of asolid polymer (for example, trade name Nation) or a perovskite typesolid oxide have been known, and used for stationary fuel cells, compactand portable fuel cells.

Performance of a proton conductor is evaluated with proton conductivity(Siemens per centimeter: S/cm). The proton conductivity represents thenumber of conducted protons per unit volume and unit time, and theproton conductivity in the temperature range employed serves as a basisfor determining whether or not the proton conductor achieves favorableperformance.

Proton conductors composed of a solid polymer currently put intopractical applications conduct proton by means of an oxonium ion (H₃O⁺)in the solid polymer. Thus, since the proton conductivity is exhibitedin the state in which water is contained in a large amount in the solidpolymer, this solid polymer is used as a solid electrolyte at atemperature not higher than 100° C. at which water evaporation isavoided.

On the other hand, since the proton conductor composed of a perovskitetype solid oxide conducts proton by hopping of proton on oxygenconstituting the solid oxide, it exhibits proton conductivity at a hightemperature of not lower than 600° C. Accordingly, this solid oxide hasbeen used as a solid electrolyte at a temperature of not lower than 600°C. In the proton conductor made of the solid oxide, the protonconductivity increases by setting the temperature in use to a highertemperature, whereas the proton conductivity decreases abruptly whenused at lower temperatures.

When the operation temperature of a fuel cell is elevated, reactionefficiency of the catalyst is enhanced, leading to enhancement ofefficiency of electric power generation; therefore, a proton conductorwhich can be used at higher temperatures has been desired. However,polymer solid electrolytes cannot be used at a temperature not lowerthan 100° C., as described above. On the other hand, there are manyrestrictions for reliability or durability of a fuel cell system for theoperation of the fuel cell at a high temperature of not lower than 600°C. Thus, realization of a proton conductor which can be used in atemperature range of about 100° C. to 400° C. has been desired.

Under such circumstances, investigations of solid electrolytes thatexhibit favorable proton conductivity in an intermediate temperaturerange of not less than 100° C. and not more than 600° C. have beencarried out (see, for example, Patent Document 1).

Patent Document 1 discloses that tin pyrophosphate SnP₂O₇ is produced byadding phosphoric acid H7PO₄ to tin oxide SnO₂ followed by heating, andthat thus obtained tin pyrophosphate exhibits high proton conductivity.

[Prior Art Document] [Patent Document]

Patent Document 1: Japanese Patent Laid-Open Publication No. 2005-294245

SUMMARY OF THE INVENTION [Problems to be Solved by the Invention]

However, although tin pyrophosphate was formed in attempts to synthesizetin pyrophosphate according to a method that is similar to the method ofPatent Document 1 by the present inventors, the raw material tin oxideremained unreacted, and to yield single-phase tin pyrophosphate wasextremely difficult. In addition, when single-phase tin pyrophosphatewas synthesized according to a method that is different from the methodof Patent Document 1, the proton conductivity was about from 10⁻⁷ S/cmto 10⁻⁵ S/cm, which was not greater than 1/1,000 as compared with theproton conductivity exhibited by proton conductors which have been putinto practical applications. Therefore, the tin pyrophosphate aloneexhibited proton conductivity that was insufficient for practicalapplications, and thus it was deemed that favorable proton conductivitywas not able to be realized.

From the foregoing, an object of the present invention is to provide aproton-conducting structure that exhibits favorable proton conductivityin the temperature range of not lower than 100° C., and a method formanufacturing the same.

[Means for Solving the Problems]

The present inventors obtained the following findings as a result ofinvestigations elaborately carried out with respect to a protonconductor that exhibits favorable proton conductivity in the temperaturerange of not lower than 100° C.

(i) A structure that exhibits extremely favorable proton conductivity inan intermediate temperature range can be obtained by mixing apyrophosphate metal salt such as tin pyrophosphate with phosphoric acid,followed by subjecting the mixture to a two-step heat treatment undercertain conditions.

(ii) The structure manufactured using tin pyrophosphate has a core ofthe tin pyrophosphate inside thereof, and at least a part of the surfaceof the core is covered by a layer containing Sn and P, the layer havinga coordination number of 0 with respect to Sn of grater than 6.

From the findings described above, the present invention wasaccomplished.

More specifically, the present invention relates to a method formanufacturing a proton-conducting structure comprising: a first heattreatment step of subjecting a mixture containing a pyrophosphate saltcontaining at least one metal element selected from the group consistingof Sn, Zr, Ti and Si, and phosphoric acid to a heat treatment at apredetermined temperature in the range of not less than 80° C. and notmore than 150° C.; and a second heat treatment step of subjecting themixture, which has been treated with heat in the first heat treatmentstep, to a heat treatment at a predetermined temperature in the range ofnot less than 200° C. and not more than 400° C.

Furthermore, the present invention also relates to a proton-conductingstructure comprising a core made of tin pyrophosphate, and a coatinglayer formed on at least a part of the surface of the core, the coatinglayer containing Sn and O, and having a coordination number of O withrespect to Sn of grater than 6.

Moreover, the present invention also relates to a method for generatingan electric power comprising the step of allowing proton to be conductedto permit electric power generation with a fuel cell comprising: anelectrolyte having a core made of tin pyrophosphate, and a coating layerformed on at least a part of the surface of the core, the coating layercontaining Sn and O, and having a coordination number of O with respectto Sn of grater than 6; and a pair of electrodes attached to the surfaceof the coating layer.

[Effects of the Invention]

According to the present invention, a proton-conducting structure thatexhibits proton conductivity providing significantly superiorperformance over single-phase tin pyrophosphate in an intermediatetemperature range of not lower than 100° C. can he obtained. Thus, apractically applicable proton-conducting structure that exhibitsfavorable proton conductivity in an intermediate temperature range ofnot lower than 100° C. at which conventional proton conductors were notable to be used can be realized.

[BRIEF DESCRIPTION OF THE DRAWINGS]

FIG. 1 shows a conceptual diagram illustrating a construction of aproton-conducting structure according to an embodiment of the presentinvention;

FIG. 2 shows a flow chart illustrating a method for manufacturing theproton-conducting structure according to an embodiment of the presentinvention;

FIG. 3 shows a view illustrating results of measurement of X-raydiffraction on the proton-conducting structure of the present invention,and on tin pyrophosphate;

FIG. 4 shows a view illustrating results of measurement of the protonconductivity in the proton-conducting structure of the presentinvention, and in tin pyrophosphate;

FIG. 5 shows a view illustrating results of measurement of DTA on themixture of raw materials of the present invention;

FIG. 6 shows a view illustrating first derivation of results ofmeasurement of DTA on the mixture of the raw materials of the presentinvention;

FIG. 7 shows a view illustrating results of measurement of DTA on themixture of raw materials of Comparative Example, and the mixture of theraw materials of the present invention subjected to a heat treatmentstep 22;

FIG. 8 shows a view illustrating a relationship between the presettemperature in the heat treatment step 22, and the coordination numberof O with respect to Sn; and

FIG. 9 shows a view illustrating results of measurement of protonconductivity in a proton-conducting structure obtained according to themanufacturing method of the present invention, and in a pyrophosphatemetal salt.

FIG. 10 shows a fuel cell comprising a solid electrolyte membrane 1composed of the proton-conducting structure of the present invention, acathode 3, and an anode 2.

FIG. 11 shows a fuel cell in which the proton-conducting structure ofthe present invention is used as an electrolyte coating acatalyst-holding particle 5 in a cathode 3 and an anode 2.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be describedwith reference to drawings.

Embodiment

FIG. 1 shows a conceptual diagram of a construction of aproton-conducting structure according to an embodiment of the presentinvention.

As shown in FIG. 1, with regard to a proton-conducting structure 10 thatis a solid electrolyte, the surface of a core 11 made of tinpyrophosphate is covered with a coating layer 12 containing Sn and O.The coating layer 12 may cover the entire surface of the core 11.Alternatively, as shown in FIG. 1, the coating layer 12 may cover only apart of the surface of the core 11. Namely, the surface of the core mayhave a region not covered by the coating layer.

The shape and the size of the core 11 made of tin pyrophosphate are notparticularly limited. However, in order to increase the contact areawith proton to achieve favorable proton conductivity, the core 11 ispreferably powdery.

The coating layer 12 in the present invention contains Sn and O asconstitutive elements. Additionally, in the coating layer 12, thecoordination number of O with respect to Sn (number of O atoms per oneSn atom) is greater than 6. The coordination number of O with respect toSn can be readily determined by measuring the structure according to anX-ray absorption spectroscopic method, particularly a conversionparticle yield process. According to this procedure, the coordinationnumber can be observed from the sample surface to the interior with adepth of about 100 nm.

The tin pyrophosphate has a theoretical coordination number of O withrespect to Sn of 6, whereas the found value is less than 6. From thisresult, it is concluded that the coating layer 12 made of a materialcontaining Sn and O other than tin pyrophosphate is formed on thesurface of the core 11 made of tin pyrophosphate in theproton-conducting structure of the present invention.

Although details of the material that may form the coating layer 12 areunknown, the material is believed to contain tin oxide since a highcoordination number of O with respect to Sn is exhibited. Althoughphosphoric acid is used for forming the coating layer 12 as describedlater, phosphoric acid will be almost sputtered since a heat treatmentstep is conducted at a high temperature in the formation. Thus, in thecoating layer 12, phosphoric acid is not contained as a principalconstitutive element. However, a slight amount of phosphoric acid may becontained. Since the constituent material of the coating layer cannot beobserved with the X-ray diffraction as described later, it is believedthat the coating layer does not have a crystal structure but is in anamorphous state.

Since a theoretical maximum coordination number of O with respect to Snis 12, the coordination number of O with respect to Sn exhibited by thecoating layer 12 is less than 12. In order to achieve more superiorproton conductivity, the coordination number of O with respect to Snexhibited by the coating layer 12 preferably falls within the range of 7to 8.5.

The thickness of the coating layer 12 is not particularly limited, andmay vary greatly depending on the place as shown in FIG. 1, but istypically not less than O to about several hundred nm.

The proton-conducting structure of the present invention is a solidelectrolyte. When the core 11 is powdery, the proton-conductingstructure of the present invention is preferably molded into apredetermined shape such as, for example, pellet or sheet in order tofacilitate the handling.

FIG. 2 shows a flow chart illustrating a method for manufacturing aproton-conducting structure in an embodiment of the present invention.

According to the present invention, the proton-conducting structure canbe prepared by, for example, the method as shown in FIG. 2. As a rawmaterial, a pyrophosphate salt containing at least one metal elementselected from the group consisting of Sn, Zr, Ti and Si, i.e., at leastone selected from the group consisting of tin pyrophosphate, zirconiumpyrophosphate, titanium pyrophosphate and silicon pyrophosphate can beused. When tin pyrophosphate is used, the proton-conducting structure ofthe present invention described above can be obtained. When any one ofzirconium pyrophosphate, titanium pyrophosphate and siliconpyrophosphate is used, the resulting proton-conducting structure isbelieved to have a structure similar to the structure of theproton-conducting structure of the present invention. In other words, itis believed that a proton-conducting structure is formed which comprisesa core made of any one of zirconium pyrophosphate, titaniumpyrophosphate and silicon pyrophosphate, and a coating layer formed onat least a part of the surface of the core, the coating layer containingO and any one of Zr, Ti and Si. It is to be noted that the coating layeris formed depending on the substance of the aforementioned core;therefore, when the core is made of zirconium pyrophosphate, forexample, the coating layer will contain Zr and O. Alternatively, theaforementioned core may contain not only any one of zirconiumpyrophosphate, titanium pyrophosphate and silicon pyrophosphate, butarbitrary two of the three substances, or all the three substances maybe contained. In addition, the aforementioned coating layer may containnot only one of Zr, Ti and Si depending on the type of the material ofthe aforementioned core, but arbitrary two of the three substances, orall the three substances may be contained.

Hereinafter, the manufacturing method of the present invention will bespecifically explained.

First, in a treatment step 20, the aforementioned pyrophosphate metalsalt, preferably the powder thereof is mixed with phosphoric acid(H₃PO₄). As the phosphoric acid, either a pure form or an aqueoussolution may be used. Although the conditions in mixing are notparticularly limited, it is preferred to stir sufficiently after bothcomponents are combined such that the surface of the pyrophosphate metalsalt is evenly brought into contact with phosphoric acid. The ratio ofthe phosphoric acid to the pyrophosphate metal salt used is notparticularly limited. The ratio can be adjusted appropriately dependingon the surface area of the pyrophosphate metal salt. However, when theamount of the phosphoric acid used is too small, the amount of formationof the coating layer becomes so low that sufficient proton conductivitycannot be secured. Therefore, it is preferred that the molar ratio ofphosphorus in the phosphoric acid relative to the metal (Sn, Zr, Ti, Si)in the pyrophosphate metal salt is approximately 0.1 to 0.7.

Although the pyrophosphate metal salt having a variety of shapes may beused, when the powder thereof is used, the particle size may be on amicron order such as, for example about 0.1 to 10 micron.

When the pyrophosphate metal salt is a powder, the mixture obtained inthe treatment step 20 may be molded into a predetermined shape such aspellet or sheet using a general mold processing machine. The molding inthis stage is preferred since it can be easily carried out. The moldedmixture is subjected to a treatment in the following heat treatment step21.

The mixture obtained in the treatment step 20 is subjected to a heattreatment by maintaining under temperature conditions of not less than80° C. and not more than 150° C. in the heat treatment step 21. In thisstep, the heating temperature may fall within the range of not less than80° C. and not more than 150° C., and may be maintained constant withinthis range. Alternatively, the temperature may be altered continuouslyor stepwise within this range. The time period of the heat treatmentcarried out may be adjusted appropriately, but for example, thetreatment may be carried out for about 10 min or longer, and preferablyfor about 1 hour.

The mixture subjected to the heat treatment in the heat treatment step21 is subjected to a treatment in the following heat treatment step 22.In this step, the aforementioned mixture is subjected to a heattreatment under temperature conditions of not less than 200° C. and notmore than 400° C. Accordingly, a structure that exhibits favorableproton conductivity can be obtained. The heating temperature in thisstep may fall within the range of not less than 200° C. and not morethan 400° C., and may be maintained constant within this range.Alternatively, the temperature may be altered continuously or stepwisewithin this range. The time period of the heat treatment carried out maybe adjusted appropriately, but for example, the treatment may be carriedout for about 10 min or longer, and preferably for about 1 hour.

According to the two-step heat treatment in the foregoing, a structurethat exhibits favorable proton conductivity is obtained. Since it wasdemonstrated that some sort of reaction processes proceed sequentiallyin the temperature range of not less than 80° C. and not more than 150°C., and in the temperature range of not lower than 200° C. as describedlater, it is inferred that a sort of reactions caused between thesurface of the pyrophosphate metal salt and phosphoric acid leads toformation of a coating layer containing a metal (Sn, Zr, Ti, Si) and Oon the surface, and this coating layer promotes conduction of proton.

It is to be noted that when tin oxide or a simple oxide such as silicaor alumina is used in place of the pyrophosphate metal salt as a rawmaterial in the manufacturing method of the present invention, astructure that exhibits favorable proton conductivity cannot beobtained.

The proton-conducting structure of the present invention may be used asan electrolyte in a fuel cell. The fuel cell is constructed to comprisethe electrolyte, and a pair of electrodes disposed to be in contact withthe aforementioned coating layer. This fuel cell executes electric powergeneration by conducting proton via the electrolyte.

Specifically, as shown in FIG. 10, a fuel cell comprises a solidelectrolyte membrane composed of the proton-conducting structure 1 ofthe present invention, a cathode 3, and an anode 2. Oxygen and Hydrogenare supplied respectively to the cathode 3 and the anode 2 to generatean electric power. Needless to say, the cathode 3 and the anode 2interpose the solid electrolyte membrane.

As shown in FIG. 11, the proton-conducting structure may be used as anelectrolyte coating a catalyst-holding particle in a cathode 3 and ananode 2. Specifically, a fuel cell comprising a solid electrolytemembrane 1, a cathode 3, and an anode 2 is prepared. At least either ofthe cathode 3 and the anode 2 comprises a catalyst-holding particle 5coated with the electrolyte composed of the proton-conducting structure6 of the present invention. The solid electrolyte membrane 1 is notlimited. However, it may be composed of the proton-conducting structureof the present invention. Oxygen and Hydrogen are supplied respectivelyto the cathode 3 and the anode 2 to generate an electric power. It ispreferred that the catalyst-holding particle 5 is composed of a carbonparticle holding a catalyst 4 such as platinum on its surface.

EXAMPLES

Hereinafter, the present invention will be explained in more detail byway of Examples, and the present invention is not limited to theseExample. The process for production may be carried out with anyappropriate modification as long as the spirit of the present inventionis not altered.

Example 1

The proton-conducting structure of the present invention wasmanufactured according to the following method.

Powdery (about 1 μm) tin pyrophosphate (manufactured by Nippon ChemicalIndustrial Co., Ltd.) in an amount of 0.9807 g, and H₃PO₄ (manufacturedby Wako Pure Chemical Industries, Ltd., a conc. phosphoric acid aqueoussolution of 85% concentration) in an amount of 0.1931 g were weighedsuch that the molar ratio of Sn:P became 1:0.5. Then, both componentswere placed into an agate mortar, and mixed by stirring for 30 min.

Thus obtained mixture was filled into a die, and molded to a pellet formusing a uniaxial press molding machine at a pressure of 100 kg/cm².

The molded mixture was maintained while heating in an electric furnaceat a temperature of 120° C. for 1 hour (heat treatment step 21), andthen maintained while heating at a temperature of 400° C. for 1 hour(heat treatment step 22). Accordingly, a proton-conducting structure wasprepared.

(Structure Evaluation)

The coordination number of O with respect to Sn in the vicinity of thesurface of the proton-conducting structure obtained in Example 1 wasdetermined according to the following procedure.

The coordination number of O with respect to Sn in the vicinity of thesurface of the proton-conducting structure was measured according to aconversion particle yield process that enables the coordination numberof O with respect to Sn to be observed from the surface of the sample tothe depth of about 100 nm. Specifically, using a conversion particleyield process that is one procedure of X-ray absorption spectroscopicmethods, an X-ray absorption spectroscopic value in the range of from4,290 eV to 5,400 eV involving 4,467 eV that represents the energy at anSnL1-shell absorption edge was measured to observe the coordinationnumber. As a result, the coordination number of O with respect to Sn was8.5. Thus, it was ascertained that the coordination number of O withrespect to Sn was grater than 6 in the vicinity of the surface of theproton-conducting structure.

Using a similar procedure, tin pyrophosphate (manufactured by NipponChemical Industrial Co., Ltd.) as a raw material was subjected to themeasurement, and as a result, the coordination number of O with respectto Sn being 5.2 was demonstrated. It is to be noted that tinpyrophosphate usually has an octahedral structure of SnO₅, and thecoordination number of O with respect to Sn is theoretically 6. Theabove found value is believed to be lower than the theoretical value dueto oxygen deficiency in the vicinity of the surface.

From the foregoing, it was verified that a layer made of a material thatis different from tin pyrophosphate was formed with the coordinationnumber of O with respect to Sn being more than 6, in the vicinity of thesurface of the proton-conducting structure of the present invention.

In order to confirm as to whether or not the structure was altered aftertin pyrophosphate used as the raw material of the proton-conductingstructure of the present invention was subjected to the heat treatmentstep 21 and the heat treatment step 22, measurement of an X-raydiffraction was carried out on both the proton-conducting structure ofthe present invention and tin pyrophosphate. FIG. 3 shows the results.In FIG. 3, the upper chart shows the result of the measurement on theproton-conducting structure of the present invention, whereas the lowerchart shows the result of the measurement on the tin pyrophosphate. FIG.3 suggests that both components gave equivalent diffraction peaks in therange of the diffraction angle of from 10° to 90°. Accordingly, it wasascertained that the proton-conducting structure of the presentinvention retained the identical crystal structure to that of tinpyrophosphate.

From the results of the measurement with the X-ray absorptionspectroscopic method according to the above conversion particle yieldprocess, and the measurement of an X-ray diffraction, it was proven thatthe proton-conducting structure of the present invention had as the core11 tin pyrophosphate having a crystal structure retained, and on atleast a part of the surface of the core 11 the coating layer 12 formedhaving an amorphous structure, in which the coating layer 12 had acoordination number of O with respect to Sn of greater than 6, and wasmade of a material that is different from tin pyrophosphate.

(Evaluation of Proton Conductivity)

Evaluation of proton conductivity on the proton-conducting structureobtained in Example 1 was made using a four-terminal conductivitymeasurement apparatus by measuring the impedance. An electrode wasformed directly on the surface of the structure by allowing for vapordeposition of platinum and palladium on the front face and the back faceof the proton-conducting structure in the pellet form. After a gold wirewas adhered on the formed electrode using a silver paste, themeasurement was carried out under an experimental condition of themeasuring frequency of from 0.1 Hz to 10 MHz, and a voltage magnitude of2 V, in a temperature range of from the room temperature to 250° C.

FIG. 4 shows the results of the temperature dependency of the resultingproton conductivity. In FIG. 4, “C” indicates the measurement results onthe proton-conducting structure of Example 1, whereas “D” indicates themeasurement results under the same condition on the tin pyrophosphatemolded into a pellet form. It should be noted that the horizontal axisin FIG. 4 represents values derived by converting the measurementtemperature to a Kelvin unit, and then multiplying its reciprocal numberby 1,000. From these results, it was verified that the proton-conductingstructure of the present invention exhibited significantly higher protonconductivity as compared with tin pyrophosphate in the temperature rangeof from the room temperature to 250° C.

From the foregoing results, significant improvement of the protonconductivity was ascertained as compared with tin pyrophosphate,according to the proton-conducting structure having tin pyrophosphate asthe core 11, which had a coating layer 12 formed on at least a part ofthe surface thereof, the coating layer 12 having a coordination numberof O with respect to Sn of greater than 6.

In order to confirm the reproducibility of the aforementionedexperiment, similar procedure was used to manufacture aproton-conducting structure, and the measurement of the protonconductivity was similarly carried out. As a result, the protonconductivity was 0.15 S/cm at a measurement temperature of 100° C., and0.23 S/cm at a measurement temperature of 200° C.

From the foregoing results, it was proven that the proton-conductingstructure of the present invention had superior proton conductivity andhigh reproducibility in the temperature range of not lower than 100° C.

(Analysis of Reaction Steps)

In order to clarify the conditions for manufacturing theproton-conducting structure of the present invention, a differentialthermal analysis (DTA measurement) was carried out.

FIG. 5 shows the results of the DTA measurement carried out using amixture of tin pyrophosphate and H₃PO₄.

Powdery (about 1 μm) tin pyrophosphate (manufactured by Nippon ChemicalIndustrial Co., Ltd.) and H₃PO₄ (manufactured by Wako Pure ChemicalIndustries, Ltd., a conc. phosphoric acid aqueous solution of 85%concentration) were mixed such that the molar ratio of Sn:P became 1:0.5to produce the mixture described above. In order to adjust to theaforementioned molar ratio, 0.9807 g of tin pyrophosphate, and 0.1931 gof H₃PO₄ were weighed. Both components were placed into an agate mortar,and mixed by stirring for 30 min. With respect to thus obtained mixture,the DTA measurement was carried out by rising the temperature from theroom temperature to 800° C. at a constant rate.

The results reveal that an endothermic reaction proceeded in thetemperature range of lower than 80° C. The progress of this endothermicreaction results from evaporation of water which had been contained inthe conc. phosphoric acid aqueous solution. In addition, it is revealedthat an exothermic reaction proceeded at a temperature of not lower than80° C. since the differential thermocouple voltage value increased.Namely, it is believed that a sort of a synthetic reaction caused inthis temperature range.

FIG. 6 shows results of first derivation of the results of the DTAmeasurement shown in FIG. 5.

As is shown in FIG. 6, peaks were present in the range of from 80° C. to150° C., and additional peaks are present at 200° C. and 630° C. in thestill higher temperature range. From these results, it is revealed thatthe synthetic reaction proceeded in the range of from 80° C. to 150° C.,whereas another synthetic reaction starts again at 200° C.

From the foregoing results, it is proven that there are two kinds ofsynthetic reaction processes in the temperature range of from 80° C. to150° C., and the temperature range of not lower than 200° C., accordingto the method for manufacturing the proton-conducting structure of thepresent invention.

FIG. 7 shows the results of the DTA measurement on Comparative Example.In FIG. 7, “A” indicates the results of the DTA measurement on a mixtureobtained by mixing tin oxide SnO₂ and H₂PO₄ (manufactured by Wako PureChemical Industries, Ltd., a conc. phosphoric acid aqueous solution of85% concentration) at a rate to give the molar ratio of Sn:P of 1:3.Whereas, “B” indicates the results of the DTA measurement on a mixtureobtained by mixing powdery (about 1 μm) tin pyrophosphate (manufacturedby Nippon Chemical Industrial Co., Ltd.) and H₃PO₄ (manufactured by WakoPure Chemical Industries, Ltd., a conc. phosphoric acid aqueous solutionof 85% concentration) at a rate to give the molar ratio of Sn:P of1:0.5, and further subjecting the mixture to a heat treatment step at400° C. for 1 hour thereafter.

From these results, it was revealed that the mixture of SnO₂ and H₃PO₄gave a peak at around 200° C.; however, an endothermic reactionproceeded in the entire temperature range. Furthermore, it was revealedthat the sample subjected only to the heat treatment step 22 withoutsubjecting to the heat treatment step 21 also caused an endothermicreaction in the entire temperature range.

From the foregoing results, it was proven that even at the sametemperature conditions, the reaction process when tin pyrophosphate andphosphoric acid were used as raw materials was completely different fromthe reaction process when tin oxide and phosphoric acid were used as rawmaterials. Moreover, the mixture prepared by using tin pyrophosphate andphosphoric acid as raw materials and subjecting to the heat treatmentstep 22 was also confirmed to go through different reaction processes.

Example 2

In order to study the upper limit of the temperature conditionsapplicable to the heat treatment step 22 in the method for manufacturingthe proton-conducting structure of the present invention, the structurewas manufactured in a similar procedure to Example 1 except that thepreset temperature at the heat treatment step 22 in Example 1 waschanged to 200° C., 300° C., or, 500° C., 600° C. The protonconductivity was measured on thus produced samples according to theaforementioned procedure. The proton conductivity at a measurementtemperature of 100° C. or 200° C. on each sample is shown in Table 1 incombination with the results of Example 1.

TABLE 1 Relationship between preset temperature at heat treatment step22 and proton conductivity (S/cm) Preset temperature at heat treatmentstep 22 200° C. 300° C. 400° C. 500° C. 600° C. (the present (thepresent (the present (Comparative (Comparative invention) invention)invention) Example) Example) Proton conductivity 0.120 0.096 0.130 0.0060.002 (measured at 100° C.) Proton conductivity 0.191 0.159 0.264 0.0150.002 (measured at 200° C.)

From the foregoing results, it was ascertained that when the heattreatment step 22 was carried out at not less than 200° C. and not morethan 400° C., the obtained structure exhibited favorable protonconductivity, and that when the preset temperature at the heat treatmentstep 22 is not lower than 500° C., the proton conductivity remarkablydecreased.

In addition, with respect to the structure obtained by changing thetemperature at the heat treatment step 22 to 300° C. or 600° C., thecoordination number of O with respect to Sn was measured according tothe aforementioned conversion particle yield process.

As a result, the sample obtained by setting the temperature at the heattreatment step 22 to 300° C. had the aforementioned coordination numberof 7.0. From this result, formation of the coating layer 12 having acoordination number of O with respect to Sn of grater than 6 wasconfirmed, similarly to Example 1.

On the other hand, the sample obtained by setting the temperature at theheat treatment step 22 to 600° C. had the aforementioned coordinationnumber of 5.5, which was comparative to the value of tin pyrophosphate(5.2). Accordingly, it was verified that the coating layer 12 in thepresent invention was not formed when the temperature at the heattreatment step 22 was a high temperature of 600° C.

FIG. 8 shows the foregoing results in connection with the coordinationnumber of O with respect to Sn in combination with the results ofExample 1.

From the foregoing results, it was proven that a proton-conductingstructure that exhibited favorable proton conductivity was manufacturedby mixing tin pyrophosphate and phosphoric acid as raw materials, andcarrying out the heat treatment step 21 of heating at a temperature ofnot less than 80° C. and not more than 150° C., followed by the heattreatment step 22 of heating at a temperature of not less than 200° C.and not more than 400° C. Additionally, it was confirmed that theproton-conducting structure had a structure containing tin pyrophosphateas the core 11, and the coating layer 12 formed on at least a part ofthe surface thereof, the coating layer 12 having a coordination numberof O with respect to Sn of grater than 6.

Example 3

A structure was manufactured in a similar manner to Example 1 exceptthat titanium pyrophosphate, silicon pyrophosphate, or zirconiumpyrophosphate (all manufactured by Nippon Chemical Industrial Co., Ltd.)was used in place of tin pyrophosphate in Example 1. However, the weightof each pyrophosphate metal used was changed such that the molar ratioof metal in the pyrophosphate metal salt: P became 1:0.5. With regard tothe obtained each structure, the proton conductivity was measured underthe aforementioned conditions in the temperature range of from the roomtemperature to 250° C.

FIG. 9 shows the results of the temperature dependency of the protonconductivity thus obtained. In FIG. 9, the measurement results on themanufactured structure are shown in combination with the measurementresults on each single-phase pyrophosphate metal salt molded into apellet form under the same conditions. From these results, it isdemonstrated that each structure exhibited significantly higher protonconductivity as compared with each pyrophosphate metal salt.Accordingly, it was revealed that even when titanium pyrophosphate,silicon pyrophosphate, or zirconium pyrophosphate was used in place oftin pyrophosphate, a structure can be manufactured that achievesremarkably superior proton conductivity as compared with that achievedby each pyrophosphate metal salt in the temperature range of from theroom temperature to 250° C.

From the foregoing results, the proton-conducting structure of thepresent invention, and the proton-conducting structure obtained by themanufacturing method of the present invention are most suitable as asolid electrolyte for use in fuel cells in an intermediate temperaturerange of not lower than 100° C.

Namely, when the solid electrolyte layer of the present invention isused in a fuel cell, more favorable specific conductance is attained,and thus improvement of the rate of electric power generation of fuelcells is expected.

INDUSTRIAL APPLICABILITY

The proton-conducting structure according to the present invention, andthe proton-conducting structure obtained by the manufacturing method ofthe present invention have superior proton conductivity, and are usedfor a solid electrolyte for use in fuel cells or hydrogen sensors.

DESCRIPTION OF NUMERALS AND SIGNS

11 Core

12 Coating layer

21 First heat treatment step

22 Second heat treatment step

A Results of DTA measurement on the mixture of tin oxide and phosphoricacid

B Results of DTA measurement on the mixture containing tin pyrophosphateand phosphoric acid, which had been subjected to heat treatment step 22

C Proton conductivity exhibited by proton-conducting structure ofExample 1

D Proton conductivity exhibited by tin pyrophosphate

1-11. (canceled)
 12. A method for generating an electric power, themethod comprising steps of: (a) preparing a fuel cell; wherein the fuelcell comprises a cathode, an anode, and a solid electrolyte membrane,the solid electrolyte membrane comprises a core and a coating layer, thecore is made of tin pyrophosphate, the coating layer is formed on atleast a part of the surface of the core, the coating layer contains Snand O, the coordination number of O with respect to Sn is greater than 6and less than 12, and (b) supplying oxygen and hydrogen to the cathodeand the anode, respectively, to generate the electric power.
 13. Themethod according to claim 12, wherein the step (b) is conducted at atemperature of not less than 100 Celsius degrees and not more than 200Celsius degrees.
 14. A method for generating an electric power, themethod comprising steps of: (a) preparing a fuel cell; wherein the fuelcell comprises a cathode, an anode, and a solid electrolyte membrane,the solid electrolyte membrane comprises a core and a coating layer, thecathode comprises a plurality of catalyst-holding particles coated withan electrolyte, the electrolyte comprises a core and a coating layer,the core is made of tin pyrophosphate, the coating layer is formed on atleast a part of the surface of the core, the coating layer contains Snand O, the coordination number of O with respect to Sn is greater than 6and less than 12, and (b) supplying oxygen and hydrogen to the cathodeand the anode, respectively, to generate the electric power.
 15. Themethod according to claim 12, wherein the step (b) is conducted at atemperature of not less than 100 Celsius degrees and not more than 200Celsius degrees.
 16. A method for generating an electric power, themethod comprising steps of: (a) preparing a fuel cell; wherein the fuelcell comprises a cathode, an anode, and a solid electrolyte membrane,the solid electrolyte membrane comprises a core and a coating layer, theanode comprises a plurality of catalyst-holding particles coated with anelectrolyte, the electrolyte comprises a core and a coating layer, thecore is made of tin pyrophosphate, the coating layer is formed on atleast a part of the surface of the core, the coating layer contains Snand O, the coordination number of O with respect to Sn is greater than 6and less than 12, and (b) supplying oxygen and hydrogen to the cathodeand the anode, respectively, to generate the electric power.
 17. Themethod according to claim 12, wherein the step (b) is conducted at atemperature of not less than 100 Celsius degrees and not more than 200Celsius degrees.